Oh, and don't be afraid to touch your components running. It can still be hard to tell what's acting up by sound even with the cover off. If you feel something that seems like its vibrating too hard; there you go. Also, if you touch something and it quiets down, that will let you know as well. That might also mean you just need to tighten it down.

Since noise vibrations resonate everything around it and if you are still uncertain which component is the problem, take a 2 or 3 foot piece of flexible tubing that's about 1/4 or 5/8 inch in diameter and carefully stick one end in your ear and then use it like a stethoscope.

That's how I found an LP gas leak that was filling the basement up with super fun combustible fuel. There was a speck of a hole in a copper pipe just as the pipe enters the house.

Only times I've had noise at startup which soon resolved itself.... was due to lack of PSU fan lubrication. Sometimes works to squirt some silicone spray into small opening between the fan hub and its housing... otherwise, just replace the PSU. (Wouldn't recommend trying to replace the PSU fan unless you know your way around PSUs and capacitors.... even then, likely not worth the risk for the potential money to be saved.)